The invention described herein was made or conceived in the course of, or under a contract with, the United States Department of Energy.
1. Field
This invention relates to improvement in superconducting wires and, in particular, to means for improving the strain characteristic of such wires.
2. Prior Art
Twenty-four metallic elements and more than 1,000 alloys exhibit a characteristic known as superconductivity. These materials, when cooled to near zero degrees Kelvin, exhibit zero resistivity. The zero resistivity characteristic of many superconducting materials is lost when the material is subject to a strong magnetic field, making them unsuitable for use in the production of electromagnets.
A number of compounds, including the intermetallics niobium-tin (Nb.sub.3 Sn) and vanadium-gallium (V.sub.3 Ga), have been found to retain their superconducting properties in the presence of magnetic fields above 200 kilogauss.
Since there is no ohmic loss in a material exhibiting zero resistivity, a small gauge superconducting wire is capable of carrying high currents. Consequently, very compact, high field strength electromagnets are now possible. These electromagnets are considerably smaller than comparable devices employing copper windings and water cooling.
Superconductors such as niobium-tin and vanadium-gallium are brittle and can be easily damaged, especially when drawn into fine filamentary wire and wound for use in electromagnets. To produce a servicable wire, the superconducting filament is provided with supporting material, usually referred to as a matrix.
An example of a superconductor, supported by a matrix, is a matrix of bronze surrounding a center filament of a metal such as niobium. During a heat treating step in the fabrication of the wire, the niobium reacts with the tin in the bronze to form a superconducting film of niobium-tin at the interface of the bronze and niobium.
The structure of a more practical wire is similar to that described above with the exception that multiple fine filaments of niobium, rather than a single filament, are embedded in the matrix.
Typically, a matrix contains a number of niobium filaments. Each filament is surrounded by a superconducting film of niobium-tin at the interface between the niobium and the matrix.
When practical wires are made and are properly heat treated, virtually all of the niobium in the fine filaments is converted to niobium-tin; however, for the sake of brevity, the portion of niobium converted to niobium-tin will be referred to herein as the superconducting film regardless of its thickness in each case.
In addition to the two structures described above, other practical structures are possible including tapes in which the superconductor is contained within a tape matrix.
The ratio of the diameter of the matrix material to that of the filament in a single filament wire is typically in the order of two or three to one. Although much higher ratios such as ten to one may be fabricated, the lower ratios are practical from the standpoint of cost, volume and current carrying capability per unit cross section.
The bending of the superconducting wire causes the portion of the wire at the outside of the bend to be elongated or strained. The wire can also be strained by a tensile force occurring during the winding, or operation of an electromagnet. This strain extends through the matrix to the superconductor. Sufficient strain can adversely affect the current carrying capacity and even produce an irreversible reduction in current capacity.